1. Technical Field
This disclosure relates to a semiconductor device which includes a switching element and a diode element, and is used in a step-up DC-DC converter, and a step-up DC-DC converter including the semiconductor device.
2. Description of the Related Art
Portable electronic devices, as typified by cellular phones, are applied to various fields at an explosive pace, and have been widely accepted. The portable device is driven by batteries. When a voltage higher than the output voltage of the batteries is required by the portable device, it is necessary to use a step-up transformer circuit. Usually, the step-up transformer circuit can be formed by a step-up DC-DC converter. For example, Japanese Laid-Open Patent Application No. 9-84333 (hereinafter referred to as “reference 1”) and Japanese Laid-Open Patent Application No. 2001-154627 (hereinafter referred to as “reference 2”) disclose step-up DC-DC converters.
FIG. 32 is a circuit diagram illustrating an example of the step-up DC-DC converter.
The step-up DC-DC converter shown in FIG. 32 includes an inductor (coil) 201, a diode element 203, a switching element 205, and a capacitor 207. An end of the coil 201 is connected to a DC (direct current) power supply 209, and another end of the coil 201 is connected to an anode of the diode element 203. One end of the switching element 205 is connected to a connection point A between the coil 201 and the diode element 203, and the other end of the switching element 205 is connected to ground (GND). One end of the capacitor 207 is connected to a cathode of the diode element 203, and the other end of the capacitor 207 is connected to ground (GND). The cathode of the diode element 203 is connected to an output terminal B.
When the switching element 205 is turned ON, an electrical current flows through the DC power supply 209, the coil 201, the switching element 205, and to ground (GND). For example, if the resistance of the switching element 205 is 0Ω (ohm), the voltage at the connection point A is 0 V, a reverse bias is applied on the diode element 203, and the voltage induced on the capacitor 207 is output.
When the switching element 205 is turned OFF, a back electromotive force occurs on the two ends of the coil 201, and a voltage higher than an input voltage is induced at the connection point A. At this moment, a forward bias is applied on the diode element 203 and a current flows through the DC power supply 209, the coil 201, the diode element 203, and the output terminal B.
By switching ON and switching OFF the switching element 205 of the step-up DC-DC converter repeatedly, an output voltage higher than the input voltage can be extracted from the step-up DC-DC converter.
Generally, as disclosed in reference 2, the switching element 205 may be a MOS transistor, the diode element 203 may be an external part, and for example, may be a semiconductor device having a built-in Schottky diode.
In the step-up DC-DC converter as shown in FIG. 32, when the switching element 205 is switched ON, the voltage at the connection point A is basically 0 V, and the voltage at the output terminal B is at a high level. However, this may cause reverse bias leakage in the diode element 203. Especially, when the diode element 203 is formed of a Schottky diode, in which a metallic element is connected to a semiconductor layer, a reverse voltage leakage current may become large.
In addition, since the back electromotive force on the coil 201 is proportional to a current change per unit time, it is required that the switching element 205 be capable of high speed switching.
Further, when the switching element 205 is switched OFF, the voltage at the connection point A and the output terminal B is at a high level, and a high voltage reverse bias is applied on the drain of the switching element 205, which is formed from a MOS transistor. Due to this, when junction leakage and off leakage (off state source-to-drain leakage) occur, the voltage at the connection point A may decrease gradually.
The above problems may reduce conversion efficiency of the DC-DC converter.